May-June 2009 GSA Bulletin Media Highlights

Boulder, CO, USA - GSA Bulletin covers various aspects of volcanism, including North Sister volcano, central Oregon Cascades; Kilauea in Hawai'i; areas around the Yellowstone hotspot; the hot-spot-derived Easter Island; and fossil evidence of hot magma backarcs in Iran. Papers also discuss the San Andreas fault; Rheic Ocean closure and Pangea formation; floodplain geomorphology along the Little Missouri River, South Dakota; a continuation of the debate over the formation of Earth’s crust; and global geologic maps as tectonic speedometers.

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Schmidt and Grunder explore North Sister volcano, the oldest and most mafic of the Three Sisters volcanoes in the central Oregon Cascades. North Sister's less than 400-thousand-year eruptive history records two major influences on eruptive style and preservation of arc volcanoes: glacial ice and local tectonics. Severe erosion of North Sister during the last glacial period has exposed the interior of the volcanic edifice, revealing evidence of three major glaciations in numerous unconformities and subglacially erupted yellow-colored tuffs that interlayer with deposits of lava and scoria. Also found at North Sister is the largest radial dike swarm in the Cascades, where dozens of dikes fed agglutinated lava flows. Over time, these dikes change from a radial pattern to a more north-south orientation, parallel to faults associated with east-west extension of the Oregon Cascades. The eruption of the through-going Matthieu Lakes Fissure 75 to 11 thousand years ago produced a greater than 11 km long, north-south trending series of volcanic vents, lava flows, and dikes. This fissure system represents a final overprinting by regional extensional forces and a redistribution of the magma supply away from the North Sister volcanic center.

Sediment is transported from continents into deep sea by sediment-gravity flows. These submarine currents can create complex sedimentary architectures on continental margins. Using three-dimensional seismic data, Heinio and Davies describe trails of depressions that are several hundred meters across and tens of meters deep and found within channel-fills in the Espirito Santo Basin in Brazil and on the Niger Delta. Similar depressions form also above channel knickpoints and other topographical irregularities. They are interpreted to form as a sediment-gravity flow responds to a topographical irregularity by erosion and deposition locally around the irregularity. Heinio and Davies propose that the trails of depressions are formed as channel-confined sediment waves and suggest this is a ubiquitous process on continental margins.

A study by Fisk et al. shows that the "supposedly gentle" Kilauea volcano in Hawai'i is far more dangerous than previously realized. Widespread deposits of ash and larger rock fragments, overlooked by previous workers, were products of powerful explosions during A.D. 400-1000 that propelled debris more than 17 km from the summit. Fisk et al.'s study describes these deposits in detail as well as localities where they can be easily observed.

The explosive rhyolitic eruptions that define the track of the Snake River Plain-Yellowstone volcanism have produced a large volume of tephra found in late Miocene and younger basin-fill sediments throughout the western United States. Anders et al. use 40Ar/39Ar isotopic dating, paleomagnetic analysis, major- and trace-element geochemistry, and standard optical techniques to establish regional tephra correlations. They focus on tephra deposits in three Neogene basins in spatially separated areas—Grand Valley, in eastern Idaho; Jackson Hole, in northwestern Wyoming; and the Granite Mountains area, in central Wyoming. These basins have experienced relatively continuous deposition from the late Miocene to the Holocene. Anders et al. found tephra layers that directly tie the stratigraphy between all three basins. Using these correlations, they found that basins experienced discrete pulses of extension separated by long periods of relative quiescence, the dates of which are staggered between basins. These pulses of accelerated extension, along with evidence of similar pulses in other basins, present a pattern of west-to-east migration that they suggest is related to the Yellowstone hotspot. The later pulse of activity in Grand Valley and Jackson Hole corresponds to the migration of the North America plate over the tail of the Yellowstone hotspot. They speculate that the earliest pulse in each basin is related to the more rapid movement of the sublithospheric hotspot head as it spreads out from its earliest known location, where the Columbia River Plateau Flood Basalts Province initiated in southeastern Oregon, to its outermost edge under central Wyoming. Their results are consistent with this model of a plume head, though not unique to it.

Easter Island (aka Rapa Nui) is fascinating due to its remote location in the South Pacific Ocean and its cultural achievement, yielding hundreds of giant stone monoliths. Easter Island also stands out among intra-oceanic volcanic islands for certain remarkable geologic characteristics, such as its location close to the super-fast-spreading East Pacific Rise and above the Easter mantle plume. Vezzoli and Acocella provide an overview of the geological and volcanotectonic evolution of Easter Island and discuss some general conceptual models of the formation and evolution of hot-spot oceanic islands. Easter Island developed, in the past 0.7 million years, in two main stages. The first stage consisted of the build-up of three overlapping shield volcanoes, culminating in the formation of summit calderas. The second stage was characterized by widespread fissural activity along the slopes of the shields. The reconstructed geologic features and volcanic evolution suggest that Easter Island is the expression of an end-member type of hot spot, characterized by low magmatic productivity, immature rift zones and scarce sector collapses, the opposite of what is found in the Hawaiian Islands.

The San Andreas fault system in southern California was reorganized in the early Pleistocene from a system dominated by two fault zones (the San Andreas fault and the West Salton detachment fault) to a network of right-lateral faults that include the San Andreas and at least four right-lateral, strike-slip faults to the southwest. Steely et al. document the complex series of events that led to the creation of the San Felipe fault zone, one of these new strike-slip faults, and further changes that happened since 600,000 years ago. The team shows that parts of the San Felipe fault zone cut latest Pleistocene to Holocene surficial deposits, suggesting that the USGS Quaternary fault-and-fold database should be updated to include the San Felipe fault as an active structure. Creation of the San Felipe fault zone about 1.1 to 1.3 million years ago rearranged the valleys and mountains southwest of Coachella and Imperial Valley and coincided roughly with initial slip across the adjacent San Jacinto fault zone. At that time, a large sedimentary basin east of the West Salton detachment broke into smaller parts, and two new crystalline-cored mountain ranges emerged adjacent to the San Felipe fault zone. Initiation of the San Felipe fault reorganized the southern San Andreas fault system, contributed to a significant southwestward broadening of plate boundary zone, and marked a change in the dominant structural style to less extension between the Big Bend in the San Andreas fault and the Imperial fault. This dramatic and rapid change occurred less than 1.5 million years ago.

The Upper Palaeozoic collision of the Gondwana and Laurentia landmasses led to the formation of the Pangea supercontinent, once the Rheic Ocean was closed. The elongated suture resulting from the collision (the Variscan Orogen) crossed Pangea from western to eastern coasts in paleo-equatorial latitudes. The arcuate western portion of this mountain belt (the so-called Ibero-Armorican Arc) is exposed in northwest Spain, where thick Pennsylvanian successions accumulated in the marine foreland basin of the Cantabrian zone facing the emergent orogen and recording the waning stages of the closure of the Rheic Ocean. Within the Cantabrian zone, the Cuera Unit and the Picos de Europa Province were the last tectonic units involved in the collision and they contain geologic information key to the reconstruction of the last phases of the Orogeny. Based on detailed geological maps and cross sections, Merino-Tome et al. provide new data concerning development of the Ibero-Armorican Arc and analyze the geodynamic processes involved.

The roughly 50 million square miles of rock exposed across the Earth's continents have been mapped by geologists ever since the first complete geologic map of England, Wales, and Scotland was scribed and published by William Smith in 1815. Geologic maps now serve as a linchpin of undergraduate education in the earth sciences and provide information on the distribution of different ages and types of rocks and structures for resource discovery, land-use decisions, and hazards assessments. In addition to these more practical applications, Wilkinson et al. examine the utility of world-scale geologic maps for the study of the long and rich geologic history of continents and the planet as a whole. Because areas and ages of the different rock types mapped across Earth's continents ultimately relate to amounts of rock formation and to rates of vertical uplift and subsidence of rock bodies after their formation, geologic maps can be thought of as spatial plate tectonic speedometers. Data on areas and ages of rock bodies determined from two global geologic maps suggests that, over the course of Earth's history, vertical rates of crustal movement associated with plate tectonics and mountain building have occurred at a rate of about 500 yards per million years. Moreover, on the basis of information from global geologic maps, there is no compelling reason not to expect the presence of some Delaware-sized fragment of primal continental crust somewhere at the modern Earth's surface.

All exposed rocks on Earth's surface experience erosion with the fastest rates documented in rapidly uplifted mountains and slowest in extremely cold or warm deserts. The oldest previously reported exposure ages are from boulders and clasts of resistant lithologies lying at the surface and the slowest reported erosion rates are derived from bedrock outcrops or boulders that erode more slowly than their surroundings. Matmon et al. present erosion rate and exposure age data from the Paran Plains, a typical environment in the Near East where vast alluvial surfaces (100-10,000 square km) are covered by well-developed desert pavements. These surfaces may have retained their original geometry for more than 2 million years. Beryllium-10 concentrations in amalgamated desert pavement chert clasts collected from abandoned alluvial surfaces in the southern Negev, Israel (representing the Sahara-Arabia deserts), indicate simple exposure ages of 1.5-1.8 million years. The ages and rates calculated in this study are exceptional not only for their magnitude but also because they represent an extensive landform.

River floodplain formation and destruction are controlled by different flows. Miller and Friedman analyzed aerial photos from 1939 to 2003 along the Little Missouri River, North Dakota, to relate flood-plain change to flow along this relatively free-flowing river. Flood-plain destruction was caused mostly by high flows occurring every 5-10 years, while flood-plain formation was promoted by unusually low flows in the fall and winter. Younger flood-plain patches were destroyed faster than older patches. This suggests that downstream movement of contaminated sediment would occur faster than predicted by models that assume all ages of flood-plain have an equal chance of being eroded. Their flood-plain age maps were similar to forest age maps produced by counting tree-rings, demonstrating that cottonwood tree establishment along this river occurs in recent channel locations.

The evolution of the hydrocarbon-bearing deepwater fold-and-thrust belt of northwest Borneo is still under discussion. On the basis of seismic reflection data, Hesse et al. restored the evolution of the compressional structures in the deepwater domain and balanced their observations with extensional structures in the shelf region. They demonstrate that shelfal extension is not sufficient to explain the evolution of the deepwater fold-and-thrust belt off northwest Borneo. Primary control for the deepwater compression is not primarily gravity gliding but basement-driven crustal shortening. Moreover, they show that folding and thrusting is ongoing today.

The growth of Earth's continental crust is a long lasting, hotly debated topic in earth sciences. This work is based on a case study of magmatic rocks generated at the end of a 500 million years old mountain-building event in Antarctica. Results indicate that large amounts of magma can be added as hidden material at the base of the crust at the end of the collision between continental plates. This hidden material can be soon remelted to generate voluminous late-orogenic potassium-rich granites. Thus, major growth pulses of the continental crust can occur at the end of major orogenic events.

Intensive and voluminous volcanism is an anomalous feature during ocean closure and collision of continents as cessation of subduction implies a cooling magmatic system. More cool magmatic systems generally imply reduced and eventually ceasing volcanism, not high magmatic flux. Yet, Ahmadian et al. have discovered an extensive 50-35-million-year-old composite intrusive complex emplaced along the Alpine-Himalayan collision zone in Iran behind the main volcanic arc just prior to collision between the Eurasian and African plate. High magmatic flux during this time occurred in the backarc, a zone of crustal stretching and extension. Geochemical signatures of these pre-collisional rocks reflect a melt source rich in K-Sr-fluids. Buckling and bulging of choking oceanic lithosphere prior to collision caused by long-term subduction of irregular oceanic floor (e.g. oceanic plateau, extinct spreading centers and hotspot trails) must have released these fluids previously stored in abundant oceanic crustal veins and faults, but explaining high backarc magmatic flux prior to collision also requires high heat flow. Modern Pacific and southern European backarc systems do show vigorous mantle convection accompanied by high heat flow. This makes them possible analogues of fossil pre-collision systems like Iran, and also suggests that broad and enduringly hot (tens of millions of years) backarcs may in fact be the rule rather than an exception. This is a new concept which challenges existing tectonic and magmatic backarc models.

Lawton et al. document long-distance transport of sediment from the southwestern United States to the rim of the Gulf of Mexico in northeastern Mexico during Late Cretaceous and early Cenozoic time. The results of this work, based on the mineralogical composition of sandstones in the Parras and La Popa basins near Monterrey, Mexico, as well as the ages of individual zircon grains in those sandstones, are significant in that they pinpoint ancient sources for the sediment in volcanic terranes of western Mexico and basement rock sources in California, Arizona, and New Mexico. In addition to permitting reconstruction of an ancient river system that existed in the northern part of Mexico from 70 to 40 million years ago, the new insight into the composition of sands delivered to northeastern Mexico may have important economic implications by providing a means of tracing these sands across the deep Gulf of Mexico, where they may contain petroleum resources.

To provide a clock for the past half-billion years of Earth's deep time, paleontologists track the successive appearances of selected fossil species. Traditional manual methods limit the number of species that can be unambiguously placed in a reliable order. The time from about 400 to 490 million years before present, for example, is traditionally divided into 60 to 70 intervals using a few of the species belonging to the extinct group of plankton called graptolites. Sadler et al. demonstrate how computer algorithms can build a clock with at least ten times more resolving power by searching for order among all of the published information concerning more than 3,800 appearance and extinction events of graptolites collected from over 400 locations worldwide. Their method, which can be applied to other fossil groups and time intervals, incorporates radio-isotopically dated volcanic ash layers to calibrate the deep-time clock and quantify its uncertainties.

Establishing the history of a fault and whether it remains active is critical to earthquake hazard analysis. This can be accomplished by using the sediment deposited in adjacent basins to constrain the time of fault initiation, the period of maximum fault activity and consequent basin subsidence, and depositional onlap of the fault following its extinction. This approach has been applied to the Saros fault system in the eastern Gulf of Corinth region of central Greece by Mack et al. The results of this study suggest that the Saros faults are intermediate in age compared to the other major faults in the region, but have been inactive for the past several hundred thousand years.